1. Field of the Invention
The present invention relates to a method of regenerating an extrusion die for ceramic honeycomb structural bodies.
2. Statement of Related Art
As known in the art, ceramic honeycomb structural bodies are widely used in various fields, for example, as catalytic carriers for clarifying exhaust gas from internal combustion engines, as filters for removing particles from exhaust gas of diesel engines, and also as rotary-type heat exchanges. An extrusion die for extruding such ceramic honeycomb structural bodies is typically manufactured by a method wherein a hard coating comprising titanium as a major component, such as a TiCN coated layer, is formed on the surface of the base metal which is comprised of a martensite series precipitation-hardened stainless steel, by a hot CVD (chemical vapor deposition) process, as disclosed in JP-A-3-281,306 and JP-A-5-269,719 (or U.S. Pat. No. 5,256,449). The hard coating so formed serves to suppress abrasion of the die as the ceramic raw material or "body" is passed therethrough, and allows the die to be used for a prolonged period.
With reference, for example, to an extrusion die for extruding a cordierite honeycomb structural bodies having an outer diameter of 118 mm, a cell density of 400 cells per square inch and a wall thickness of 0.15 mm, as shown in FIG. 1, approximately 3,400 feeding holes 1 are formed in the base metal member 2 of the die, and each feeding hole 1 is aligned with a shaping orifice 3 and has an inner diameter of 1.0-1.5 mm and a depth of 18-36 mm. The shaping orifice 3 determines the cross-sectional shape of each cell in the honeycomb structural body. Thus, the shaping orifice 3 in the cross-section of the die is typically of a polygonal shape, e.g., a triangular, square or hexagonal shape, or of a circular shape.
FIG. 2 is an enlarged sectional view of a part of the extrusion die and illustrates a state in which the surface of the feeding hole 1 and the orifice 3 to be brought into contact with the raw ceramic material is uniformly formed with a hard coating 4 which comprises titanium as a major component. However, even an extrusion die provided with a hard coating of the above-mentioned abrasion-resistant material undergoes a progressive wear after a long period of use so that the thickness of the hard coating decreases gradually. Moreover, the amount of wear is more significant in the center region of the die, and less significant in the peripheral region thereof. Continued use of an extrusion die exhibiting a non-uniform wear amount often results in undesirable non-uniformity of the wall thickness of the honeycomb structural bodies as they are extruded form such a die, thereby giving rise to inferior product quality.
To overcome such a problem, the inventors developed and previously proposed a method wherein the peripheral region of the die exhibiting less significant wear amount is applied with a masking, and a fresh hard coating comprising titanium as a major component is then applied to the unmasked center region of the die exhibiting a significant wear amount, so as to regenerate a hard coating to have a uniform thickness over the entire region of the die, as fully disclosed in JP-A-7-256,618. This method proved to be useful effectively to regenerate worn extrusion dies so that ceramic honeycomb structural bodies extruded from such regenerated dies are substantially free from rough surface formation in their peripheral regions, thereby achieving an improved uniformity in terms of wall thickness of the honeycomb structure.